The term “inertial sensor” is used in the field of electronic sensors to encompass both accelerometers (sensors that measure linear acceleration) and gyroscopes (sensors that measure angular rate). Microelectromechanical Systems (MEMS)-based accelerometers and gyroscopes have become ubiquitous in recent years, and are often far more effective than their conventional macroscopic counterparts.
There is an array of different designs that are used in order to implement these MEMS-based accelerometers and gyroscopes. MEMS-based linear accelerometers typically use a comb-like structure comprising sets of interdigitated fixed and moveable “fingers” that can be used to sense a physical displacement therebetween, wherein the displacement is proportional to an applied linear acceleration. Capacitive, inductive or piezoelectric sensing techniques may be used.
By way of contrast, MEMS-based gyroscopes are typically implemented using vibrating structures and are often referred to in the art as “vibrating structure gyroscopes” or “VSGs”. These VSGs typically use a planar structure such as a ring or cylinder that are made to vibrate in a cos nθ mode of vibration (e.g. n=2) as discussed, for example, in EP0565384 and U.S. Pat. No. 7,637,156. Briefly, in a cos 2θ mode of vibration, every point on the vibrating structure moves radially—e.g. in a straight line from the centre of the ring—except for ‘nodes’ at 90° intervals around the structure which remain stationary. When a rotation is applied, the Coriolis force causes points on the vibrating structure that are moving radially outwards at any given point in time to “bend” in one direction, while points on the structure that are moving radially inwards at that same point in time bend in the other direction. The angular rate (e.g. measured in degrees per second) can then be determined by either detecting the amount by which these nodes move with respect to each other (known as “open loop measurement”) or by applying a restorative force to keep the structure vibrating solely in the original cos 2θ mode wherein the restorative force is proportional to the applied angular rate (known as “closed loop measurement”), nullifying any oscillatory motion in the secondary mode of vibration. Such an angular rate sensor (commonly referred to as a gyroscope) uses primary and secondary drive and pick-off transducers. A primary control loop maintains resonance of the vibrating structure by generating a 90° phase shift between the primary drive and pick-off transducers. A secondary control loop receives a signal indicative of rotation from the secondary pick-off transducer and nulls the secondary drive transducer to zero. In addition to the oscillatory signals applied to the primary and secondary drive transducers, the vibrating structure must be biased with a direct current (DC) voltage offset in order for resonance to be achieved.
Self-contained systems known as “inertial measurement units” (IMUs) containing a plurality of inertial sensors such as accelerometers and/or gyroscopes are typically used in aircraft, spacecraft, watercraft, unmanned aerial vehicles (UAVs) and guided missile systems to provide inertial navigation. For example, an IMU providing six degrees of freedom of inertial sensing may comprise three MEMS-based vibrating ring gyroscopes and a plurality of accelerometers arranged to measure linear acceleration along three axes. However, the Applicant has now appreciated that a single inertial sensor may be capable of measuring both angular rate(s) and linear acceleration(s).
The Applicant has also appreciated that typical VSGs suffer from “charge trapping”, because the direct current (DC) bias that is applied to the vibrating structure causes long term scale factor and bias drift, preventing the VSG from being used in high performance applications. This occurs when DC bias signals are used for extensive periods of time and charges may become “trapped”, altering the bias level of the VSG.